1. Field of the Invention
The invention relates to a method of optimizing the release of constituent exhaust gas that has been stored in a vehicle emission control device during xe2x80x9clean-burnxe2x80x9d vehicle operation.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-retaining efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables. Thus, both the device capacity and the actual quantity of the selected constituent gas stored in the device are complex functions of many variables.
It is an object of the invention to provide a method and system by which to optimize the purge time during which a previously-stored constituent gas of the engine-generated exhaust gas is released from a vehicle emission control device.
Under the invention, a method of optimizing a purge time for an emission control device, itself located in the exhaust passage of an engine upstream from an oxygen sensor, includes detecting the sensor output voltage; calculating a purge time correction factor related to a characteristic of the sensor output voltage, for example, based on the error between a desired saturation time value and a calculated saturation time value; and calculating a subsequent purge time as a function of the product of the correction factor and a present purge time. The optimization of purge time is preferably continued until the absolute value of the difference between present and subsequent purge time values, is less than an allowable tolerance.
In accordance with a feature of the invention, a preferred method includes sampling the sensor output voltage during a window to determine the peak voltage; and the calculated saturation time value is based on the time that the peak voltage is greater than a reference voltage. The calculated saturation time is also preferably based on the value of the peak voltage, particularly if the peak voltage is less than or equal to a predetermined reference voltage; and is otherwise preferably based on the time that the peak voltage exceeds the reference voltage.
In accordance with another feature of the invention, in a preferred method, the error is input to a feedback controller that produces the correction factor; and there is a direct, monotonic relationship between the correction factor and the error.
Similarly, under the invention, a programmed computer controls the purge time of the emission control device based on the amplitude of the voltage of a downstream oxygen sensor and the time response of the sensor.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.